1. Technical Field of the Invention
This invention relates to a contactless profiling method and, more particularly, to a contactless profiling method for tracing a model surface while performing attitude control in such a manner that the optic axis of a contactless probe, which is capable of simultaneously measuring distances to three points on the model surface, is pointed in the direction of a line normal to the model surface at all times.
2. Description of the Related Art
In conventional tracer control, a stylus is moved while made to contact a model. A tracing velocity is generated along each axis by performing tracing computations using three-dimensional axial deviations produced by tracer head contact between the stylus and the model. The model is traced, while the stylus is made to contact the model surface, in accordance with the velocity signals along the respective axes.
Because the stylus contacts the model, the stylus sustains wear and must be replaced unless the tracing velocity is made to exceed a certain value. Highly accurate tracer control will not be possible unless the stylus is not replaced. Accordingly, a contactless tracer control method has been proposed in which a model surface is traced by a distance measuring probe capable of measuring distance in contactless fashion.
FIG. 7 is a view illustrating the general features of contactless profiling. Contactless profiling refers to a profiling method in which the probe is made to trace the surface of a model MDL without contacting it by making use of a probe sensor (a distance measuring probe) PB which is capable of measuring distance without contact as in the manner of a laser distance measuring probe. The contactless-type distance measuring probe PB generally includes a reference distance L.sub.0 and is so adapted that the difference between an actually measured distance L and the reference distance L.sub.0 is capable of being output as an error quantity .sub..DELTA. L. A probe which employs, e.g., an optical triangulation distance measuring method, is available as the a contactless-type distance measuring probe. In a probe of this kind, laser light produced by a light-emitting element (a semiconductor laser) irradiates the model surface via a projecting lens, and part of the light beam scattered and reflected at such time forms a spot on a position detecting element via a light-receiving lens. The position of the spot varies depending upon the distance to the model surface, thereby allowing measurement of the distance.
Assume that the model MDL shown in FIG. 7 is to be profiled, and that points A, B, C are suitably selected sampling points (measurement points). When a distance L.sub.1 (=L.sub.0 +.sub..DELTA. L.sub.1) at point A is measured and gives an error of .sub..DELTA. L.sub.1 in comparison with the reference distance L.sub.0, the probe is directed to the next sampling point B while a corrective operation is applied along the measurement axis (along the optic axis) in an amount corresponding to the error. An error quantity .sub..DELTA. L.sub.2 is then obtained at the point B and the probe is directed to point C so as to similarly apply a correction corresponding to this error. By subsequently repeating identical processing, the model surface can be traced contactlessly while the distance L.sub.0 is maintained.
The above-described contactless-type distance measuring probe is capable of measurement in one dimension only, namely along the measurement axis. As a consequence, the larger the angle .theta. between the measurement optic axis AX and the model surface, as shown in FIG. 8(a), the more the accuracy of distance measurement deteriorates, giving rise to a range within which measurement is impossible. With the proposed contactless-type profiling method, therefore, the profiling range is limited. When the angle .theta. becomes too large, contactless profiling can no longer be applied.
In order to mitigate this limitation as much as possible, the probe PB should be rotated through a predetermined angle about its rotational axis R.sub.C in the tracing plane (e.g., the X-Z plane) to control the measurement optic axis AX in such a manner that it will lie at right angles to the model surface at all times. As shown in FIGS. 8(b) and 9, the measurement optic axis should be controlled so as to be perpendicular to the model surface even where it is cut by a plane (the Y-Z plane) perpendicular to the tracing plane. In other words, if the optic axis is controlled so as to point along the direction of the normal to the model surface at all times, distance can be measured accurately at all times irrespective of the model shape and the range over which contactless profiling is possible can be enlarged.
However, an effective method of performing contactless profiling while pointing the optic axis in the direction of a line normal to a model surface has not yet been proposed.